Simple polysaccharides, such as bacterial levan (BL), not conjugated to protein elicit a thymus independent (TI) response. Although the TI response is clonally restricted, the repertoire of antibodies that can be produced is much greater as evidenced by the appearance of new clonotypes produced in mice immunized with polysaccharide-protein complexes. The focus of this investigation is on the mechanisms or events that determine which of the potential antibodies is actually expressed. Our earlier studies examining the isoelectric focusing (IEF) pattern of IgG antibodies produced in response to immunization with BL in genetically defined mouse strains demonstrated that the complexity of the IgG response is regulated by at least one C57BL/6 gene, designated spectrotype regulation gene 1 (Sr-1), and is not linked to the Igh gene complex, MHC, or coat color. Work in progress is focused on mapping the Sr-1 gene to a murine chromosome. A large group of (BALB/c X C57BL/6)F1 mice backcrossed to BALB/c has been bred, immunized with BL, and typed as Sr-1 positive or negative by IEF analysis of anti-inulin antibodies in the serum. Spleen DNA has been isolated from all of the backcross 1 (BC1) mice and a set of genetic markers has been selected for the initial screening of the chromosomes. Genotyping of the mice is being done using simple sequence repeat (SSR)-PCR. All of the selected markers have been tested on DNA from the parental mouse strains and approximately one third of the markers were not usable for the analysis. Replacements for these genetic markers have been selected and testing of these new markers with DNA from the parental strains is currently in progress. Genotyping of the BC1 mice using markers on chromosomes 1 through 9 has been completed and thus far no linkage between Sr-1 and any of these markers has been detected. BC2 and BC3 mice have been bred to to used in later analyses. Genotyping of BC1 mice with markers on chromosomes 10 through 19 is in progress. Recent studies using TCRb/TCRd double knockout mice mice immunized with BL demonstrated that these animals produce antibodies to both BL and inulin, providing additional evidence that this response is T cell independent. Mapping and characterization of a gene (Sr-1) which regulates the expression of antibody diversity following immunization with a polysaccharide antigen will contribute to a better understanding of the immune response to polysaccharides and of the regulation of antibody diversity.